Very easy to use. Converts instantly to QuickTime. High quality video, with flexible quality settings. Smooth "mouse tracking" feature. Records audio well. Excellent process for setting capture area... you can change apps and various other normal operations, unlike SnapzPro. Numerous preferences for useful functions like setting default save folder, always showing capture rectangle, custom keyboard shortcuts, and more.
One test was recorded at medium quality, with 15fps rate and h264 compression. iShowU's options are very easy on novices, and you can create presets that the software remembers from one session to the next. However, you can't resize the video within iShowU. You'll need to open it in another app, such as QuickTime Pro, to compress and resize it.
Opened the clip in QuickTime Pro and edited it quickly down to about 30 seconds. Interestingly, QuickTime Pro remembered some of the settings I had set in DisplayEater shortly before. QuickTime Pro has excellent and easy to use export controls, so moving here for the export isn't exactly a hardship. The conversion speed was only slightly slower than DisplayEater. However, the shrunk file, again at 320 pixels wide, was 804kb, considerably larger than the DisplayEater file. Without the audio, the file shrunk to 364kb, still twice the size, even though the dimensions were smaller. I need to go do a DisplayEater demo of the same content!
Note: iShowU is designed to run in the background and will hide when not in use. It sports a handy menubar extra for starting/stopping recording, if the keyboard shortcuts don't do it for you.
Bug: iShowU recorded audio even when I had unchecked the "record audio" checkbox. [I discovered that you need to remove the microphone input device to keep iShowU from recording audio.]
I'd say iShowU is the fastest of all these tools, and is very well behaved with CPU cycles and RAM.